1. Field of the Invention
The present invention relates to a thin-film transistor substrate and a liquid crystal display device using the same, and more particularly, a thin-film transistor substrate using aluminum to form a low-resistance interconnection and a liquid crystal display device using such a thin-film transistor substrate.
2. Description of the Related Art
Because of its low resistance, aluminum is widely used in electronic devices to form interconnections and electrodes on a substrate.
FIG. 6 is a schematic diagram illustrating a thin film transistor of a type widely used in thin-film transistor liquid crystal display devices.
The thin film transistor 82 includes a gate electrode 84 formed on a transparent substrate 83 and a gate insulating film 85 formed over the gate electrode 84. A semiconductor active film 86 made of amorphous silicon (a-Si) is disposed on the gate insulating film 85 formed on the gate electrode 84. Ohmic contact layers 87 made of amorphous silicon (n+-type a-Si) including an n-type impurity such as phosphorus are disposed on the semiconductor active film 86. Source and drain electrodes 88 and 89 are disposed on the semiconductor active film via the respective ohmic contact layers 87 and they extend into areas on the gate insulating film 85. A passivation film 90 is formed in such a manner as to cover the thin-film transistor 82 composed of the source electrode 88, the drain electrode 89, the gate electrode 84, and other elements. A contact hole 91 is formed in the passivation film 90, at a location above the drain electrode 89. There is also provided a pixel electrode 92 made of a transparent conductive film such as indium tin oxide (hereinafter referred to as ITO), which is electrically connected to the drain electrode 89 via the contact hole 91.
A part on the left of FIG. 6 illustrates a cross section of a gate terminal pad 93 of a gate interconnection located outside the displaying area. A lower pad layer 94 is formed on the transparent substrate 83 using the gate interconnection material. A contact hole 95 passing through the gate insulating film 85 and the passivation film 90 is formed at a location above the lower pad layer 94. An upper pad layer 96 is formed of the same transparent conductive film as that used to form the pixel electrode 92 and is electrically connected to the lower pad layer 94 via the contact hole 95. The end of the source interconnection is also formed in a similar structure.
In the thin film transistor, as described above, the transparent conductive film used to form the gate terminal, the source terminal, and the pixel electrode is directly connected to the interconnection metal used to form the gate interconnection, the source interconnection and the drain electrode.
In liquid crystal display devices of the above-described type employing ITO and aluminum as the materials for the transparent conductive film and the interconnection metal, respectively, to achieve low interconnection resistance, if ITO and aluminum are connected directly to each other, then aluminum is oxidized by oxygen contained in ITO. This causes an increase in contact resistance between ITO and aluminum.
A hillock is another problem which occurs when aluminum is used as the interconnection material. The hillock refers to a needle-shaped projection produced on the surface of aluminum. The hillock can extend through an insulating layer formed on aluminum until reaching a conductive layer on the insulating layer. This causes a short circuit and poor electric isolation.
In view of the above, the object of the present invention is to provide a thin-film transistor substrate and a liquid crystal display device using such a thin-film transistor substrate which use aluminum as the interconnection material without causing the above-described two problems, that is, an increase in electric resistance at a contact between aluminum and ITO and a short-circuit or poor electric isolation.
According to an aspect of the present invention, there is provided a thin-film transistor substrate comprising: a substrate; an underlying metal film disposed on the substrate, the underlying metal film being formed of a metal capable of being electrically connected to an indium tin oxide film used to form a gate terminal, a source terminal, and a pixel electrode; an aluminum film used to form a gate interconnection, a source interconnection, and a drain electrode, the aluminum film being disposed on the underlying metal film; an aluminum oxide film disposed on the aluminum film; an insulating film disposed on the aluminum oxide film; a contact hole formed in the insulating film, the aluminum oxide film, and the aluminum film, the contact hole extending from the surface of the insulating film to the surface of the underlying metal film through the insulating film, the aluminum oxide film, and the aluminum film; and an indium tin oxide film formed on the insulating film and in the contact hole, the indium tin oxide film in the contact hole being electrically connected to the underlying metal film.
In the thin-film transistor substrate, it is required that the gate terminal, the source terminal, the pixel electrode, and other elements formed of the indium tin oxide film be electrically connected to the gate interconnection, the source interconnection, the drain electrode, or other elements formed of the aluminum film. In the present invention, instead of directly connecting the indium tin oxide film and the aluminum film to each other, the indium tin oxide film is indirectly connected to the aluminum film via the underlying metal film in such a manner as described below. The gate interconnection, the source interconnection, and the drain electrode are formed in a two-layer structure consisting of the underlying metal film and the aluminum film, and the contact hole is formed in the insulating film formed on the two-layer structure such that the contact hole further extends through the aluminum film until reaching the underlying metal film. The indium tin oxide film is then formed such that it is connected to the underlying metal film. In this technique, the underlying metal film is formed of a metal capable of being electrically connected to the indium tin oxide film. As a result, the gate terminal, the source terminal, and the pixel electrode are electrically connected to the gate interconnection, the source interconnection, and the drain electrode, respectively.
Metals which may be preferably employed to form the underlying metal film include Mo, Ti, and Cr. For example, when molybdenum is employed, it is possible to obtain a contact resistance between molybdenum and ITO in the range from 10xe2x88x924 to 10xe2x88x927 xcexa9xc2x7cm2 which is very low compared with contact resistance of 102 to 10xe2x88x922 xcexa9xc2x7cm2 obtained for contacts between aluminum and ITO.
Furthermore, the aluminum oxide film formed on the surface of the aluminum film serves as a barrier layer which prevents growth of hillocks on the surface of the aluminum film during a heat treatment or the like performed after the formation of the aluminum oxide film. Thus, short circuits and poor electric isolation due to hillocks can be effectively prevented.
As described above, the present invention solves both of the two problems: increase in resistance at a contact between the gate terminal, the source terminal, or the pixel electrode and the gate interconnection, the source interconnection, or the drain electrode; and short circuits and poor electric isolation due to hillocks on the aluminum film. As a result, it is possible to produce a thin-film transistor having good electric characteristics with a high production yield.
The aluminum oxide film may be produced by various methods. One simple method is to oxidize the aluminum film using ozone water. Another simple method is to irradiate the aluminum film with an ultraviolet ray in an oxygen ambient. The above two methods may be combined.
According to another aspect of the invention, there is provided a liquid crystal display device including: a pair of substrates opposing each other; and a liquid crystal disposed between the pair of substrate, wherein one of the substrates is a thin-film transistor substrate according to the technique of the invention described above.
In the liquid crystal display device using the thin film transistor substrate employing aluminum to form low-resistance interconnections, signal voltage drops and signal propagation delays along the interconnections due to interconnection resistance are minimized. Thus, it is possible to easily realize a display device having a large displaying area which needs interconnections extending along long lengths, or a high-density display device which needs thin interconnections.